The penumbra of irradiations in linear accelerators, its use in radiotherapy of cancer diseases, negative effects, and the possibilities of reducing them

Abstract

The aim of this study is to highlight the understanding of radiation, changes in the radiation of the same type but with different energy, the causes of these changes, the damages caused by their presence in the primary beam, or the reduction of the effectiveness of radiation, compared with the same radiation beam in which the presence of penumbra is higher. Penumbra is the region near the edge of the field margin where the dose falls rapidly. The dose falls off around the geometric beam edge that is sigmoid in shape and extends under the collimator jaws into the penumbral tail region, where there is a small component of the dose due to the transmission through the collimator jaws (transmission penumbra), a component attributed to the finite source size (geometric penumbra) and a significant component due to the in-patient X ray scatter (scatter penumbra). The total penumbra is referred to as the physical penumbra and it is the sum of the three individual penumbras: transmission, geometric and scatter. Without pretending that we can eliminate the negative effects caused by the presence of penumbra in the primary beam, we note that: a part of the quantitative reduction of the radiation dose already performed through the use of high energy of linear accelerators and further reduction of the energy difference between the primary radiation with average beam energy can significantly improve the quality of the beam, including radiation.